1. Field of the Invention:
The invention relates to nuclear fuel assemblies of the elongated type wherein nuclear reactor fuel in solid form is contained within a protective sheath, and more particularly to such a fuel assembly for use in light water reactors, wherein the assemblies are positioned with their longitudinal axes substantially parallel to the normal vertical axes, the nozzle of each fuel assembly being provided with compensators to compensate for an out-of-plumb, or skewed, position during lifting and handling, such as in loading and unloading of the reactor core.
2. Description of the Prior Art:
In recent development of replacement fuel assemblies for light water reactors, a particular fuel element design feature has resulted in asymmetric mass distribution in the radial blanket fuel to achieve advantages over present designs. Such asymmetric mass distribution, although desirable, causes the fuel to hang skewed from the handling tool, rather than plumb with respect to the central axis of the core. It has been calculated that such fuel assemblies having a length of approximately 160 inches and supported at the top nozzle would be displaced approximately 1/2 inch at the bottom from a plumb line passing through the central axis of the fuel assembly at the top. In certain positions in the radial blanket, such as at the blanket edge, a blanket edge fuel assembly will hang out of plumb in only one coordinate direction, whereas in another position, such as the blanket corner, a blanket corner fuel assembly will hang out-of-plumb in two coordinate directions.
A skewed position of the type described above could make it difficult to unload and reload this type of fuel because several fuel assemblies would have to be put in temporary positions in the core and excess fuel movements would have to be made to guide these radial blanket assemblies down on the lower core support hinge. This would increase the opportunity for fuel assembly damage during fuel insertion. Therefore, special handling precautions would have to be used with any movement of the radial blanket fuel assemblies in or out of the core. An additional problem encountered with this fuel design is increased bowing in the later life of the assemblies, due to their uneven weight distribution, which could make it extremely difficult for them to move in and out of the core through a complete life cycle. All these previously mentioned problems result in increased time consumption for extra handling precautions and increased numbers of fuel handling operations which it has been estimated could double the time it now requires to complete a refueling operation. Similar handling problems would be experienced in withdrawing the skewed fuel assemblies into the refueling machine mast which would also significantly increase refueling time. Such increases in refueling time, for example, are extremely undesirable since they increase man-rem exposure to the crew and result in longer periods of plant off-line times.
Known fuel assembly handling devices, such as that shown in U.S. Pat. No. 4,236,966, are normally insertable into the upper nozzle portion of the fuel assembly and have gripper devices on the inserted ends which grip under shoulder portion, for example, on the nozzle or end of the fuel element assembly, for handling the fuel assembly during insertion or withdrawal from a reactor core, storage basin, or fuel transfer device, for example. However, such devices have no means for compensating for fuel elements, or fuel assemblies, which would not hang vertically on the handling tool due to uneven weight distribution but instead would hang out-of-plumb, thus imposing the problems pointed out above during insertion and withdrawal.